The relative formula weight is also known as the relative molecular mass (RMM). The RMM of a compound can be found by adding up the relative atomic masses (RAM) of each of its atoms, therefore for H2CO3:
RMM = 2*RAM(H) + RAM(C) + 3*RAM(O)
= 2*1.0 + 12.0 + 3*16.0 = 62.0 g/mol